1. Field of the Invention
This invention relates to cellular telephone apparatus and, more particularly, to dual mode cellular telephone apparatus capable of operating in both analog and digital modes wherein a portable unit connectable to a mobile unit may communicate with a cellular base station in either mode.
2. Description of the Prior Art
Cellular telephone standards provide particular conventions and formats by which voice signals are communicated between mobile stations and cellular base stations. Typically, communication is carried out in analog form, wherein analog voice signals are modulated onto a carrier and transmitted either from a mobile station to a base station or from a base station to a mobile station. As an example, frequency modulation has been used for such communication.
Conventionally, a mobile station refers to a cellular telephone transmitter/receiver that moves from point to point. Typically, mobile stations are disposed in vehicles, such as automobiles, boats, etc., and rely upon the vehicle source of power to energize the electronics included in the mobile station. Heretofore, the construction of the transmit/receive/processing circuitry, the size of the usual radio antenna used therewith and the reliance on an external source of power have made it difficult to implement a mobile station in truly portable dimensions. Recently, however, advances in electronic miniaturization and improvements in circuit performance have resulted in the introduction of portable, or hand-held, mobile units.
A typical portable mobile unit includes a battery, such as a rechargeable battery, for supplying electrical energy, a microphone for converting audible signals to electrical voice signals, processing, transmitting and receiving circuitry to enable the analog transmission and reception of voice signals, a loudspeaker for converting received voice signals to audible sounds and an antenna, such as a low gain antenna, by which analog voice signals may be communicated between the portable unit and a typical cellular base station. The portable unit also includes displays to apprise the user of different modes of operation, a keypad by which the user may effect a telephone call to a desired party via the cellular communication system, and other ancillary or auxiliary elements. If the battery is of the rechargeable type, suitable external contacts may be provided on the portable unit to facilitate charging of the battery without removal thereof from the unit.
To enhance cellular telephone systems and add capabilities not heretofore exhibited by analog systems, particularly in geographic areas wherein cellular communication traffic is heavy and dense, a dual mode cellular standard, identified as the TIA IS-54 standard, has been introduced. This dual mode standard permits voice signals to be transmitted either in analog form, as before, or in digital form. The application of digital techniques to voice communication improves the quality and fidelity of the transmitted voice signals and increases the number of users which may access the cellular system simultaneously. In addition, the cellular communication system may be used to communicate other information and data and need not be limited solely to voice communication.
In the dual mode cellular standard, a mobile station is controlled to communicate in analog form in the event that the cellular system with which it is used does not exhibit digital communication capabilities, such as in exurban environments, or in the event that all of the digital channels are being used but some analog channels nevertheless are available. At other times, the mobile station is controlled to communicate in digital form. However, to provide dual analog/digital capabilities in a mobile unit generally means that additional circuitry must be provided therein, resulting in large, relatively heavy, bulky apparatus. As a consequence, the dual mode mobile unit no longer is considered portable. That is, the unit no longer is of a size and weight conducive for hand-held operation.